The present invention relates to hydraulic systems, and more specifically, to solenoid controlled hydraulic braking systems for automobiles.
Hydraulic systems typically are the basis for vehicle braking systems, especially automotive braking systems. The ability of a hydraulic system to convert fluid pressure into linear/mechanical motion, and to allow the source of the hydraulic pressure to be positioned remotely from the cylinders which effect the braking action, is particularly desirable in automotive systems. Such automotive braking systems are hydraulic throughout, consisting of an actuator, such as a brake pedal, a reservoir of fluid responsive to pressure applied by the actuator, such as a master cylinder, and means for converting the hydraulic pressure to the braking force, such as cylinders. Thus, in these standard systems, braking pressure has been achieved mechanically, utilizing the force of the depression of the brake pedal by the driver (usually accompanied by a vacuum boost) that consequently increases the pressure in the master cylinder. The increased pressure in the master cylinder is then transmitted through lines of fluid to the cylinders which operate the calipers or shoes, thereby forcing the calipers and shoes against the rotors or drums, respectively, to slow the automobile.
Motor-based antilock braking systems (xe2x80x9cABSxe2x80x9d) are frequently used with standard all hydraulic braking systems in order to prevent vehicle skidding during xe2x80x9cpanicxe2x80x9d braking events. In commonly used motor-based ABS, a motor/piston assembly is used to modulate the fluid pressure to the calipers during ABS braking events to quickly cycle the brakes between apply and release modes. In a typical ABS, a wheel speed sensor senses when individual wheels on a vehicle begin to xe2x80x9clock-upxe2x80x9d (i.e. cease rotation) during braking, which is an indication that those vehicle wheels are beginning to skid. Vehicle skidding is undesirable in that the vehicle stopping distance could be lengthened and vehicle control is lessened. Accordingly, in order to minimize skidding, the ABS modulates hydraulic fluid flow to the vehicle wheel brakes that are about to lock up, thereby causing such brakes to alternate between applying braking pressure and releasing braking pressure in a controlled manner at a high rate to optimize tire slip without allowing the wheels to lock-up.
However, during ABS operation, there is an undesirable lag in response time in the braking system due to the back pressure of the hydraulic fluid in the system that is placed on the motor. In other words, the motor does not respond as quickly as desired in resetting to a neutral position or in reversing direction in response to a signal received from the brake control requesting a change in pressure to the calipers. While some of this response lag is caused by the inertia inherently present in the motor itself, most of the problem is attributable to the back pressure of the hydraulic fluid on the internal components of the motor. Thus, motor response and or resetting lag times could be decreased, and overall ABS control improved, if the pressure load on the ABS motor was relieved as the motor is commanded by the controller to go from release to reapply. Also, by closing the solenoid and limiting the pressure to the calipers during transition from apply to release, the motor turn around time could be reduced by deadheading the motor and eliminating pressure overshoot at the wheel brake.
While control improvements such as these are useful in controlling the pressure load on ABS motors in standard all hydraulic systems, there are additional disadvantages of standard systems that are not solved by merely improving hydraulic control. Some of these disadvantages include the large amount of mass and volume that the master cylinder and the hydraulic lines add to the completed automobile. Furthermore, the number of assembly hours that are required to install standard hydraulic braking systems when combined with the large number of parts that these systems generally require, further adds to their expense and undesirability. Other disadvantages of standard hydraulic braking systems is their dependence on vacuum boost to assist in the braking operations due to the advent of vehicles, such as electric cars, which do not produce vacuum as a by-product of the vehicle operation.
Accordingly, there have been recent advances in standard hydraulic braking technology using xe2x80x9cbrake-by-wirexe2x80x9d technology to overcome some of these disadvantages. In particular, a subset of xe2x80x9cbrake-by-wirexe2x80x9d systems known as xe2x80x9cdry interfacexe2x80x9d systems has been found to be particularly useful. In a typical dry interface system, the driver input is transmitted to the system electronically through an electronic controller, rather than mechanically and hydraulically, to the braking devices at each corner. The corner is a term used in the industry to describe all of the equipment that is used at the wheel assembly, including, but not limited to, the braking, suspension, and drive train systems. These new dry interface systems ameliorate several of the aforementioned disadvantages of standard hydraulic braking systems. In particular, dry interface systems improve on standard braking systems in performance, vehicle design, assembly, and repair areas. For example, the elimination of an all-hydraulic braking system means that hydraulic lines are no longer necessary to convey brake fluid from the master cylinder to the corners of the automobile, thereby eliminating a significant amount of mass from the automobile. Furthermore, assembly costs are reduced since dry interface systems consist of individual modules having relatively few separate components. The dry interface modules merely need to be bolted to the automobile and plugged in, in contrast to the significant assembly time required for the installation of standard, fully-hydraulic braking systems. The reliability of the systems is thereby increased accordingly.
While such dry interface systems eliminate many of the hydraulic components of the standard hydraulic systems, it should be noted that these systems are not completely xe2x80x9cdry.xe2x80x9d Unlike standard hydraulic systems, which utilize vacuum-assisted driver brake pedal force to create the pressure to apply the brakes, the dry interface system utilizes individual motors that drive ballscrew piston assemblies at each corner to create the necessary hydraulic pressure for applying the brake pressure at that corner. Accordingly, while assembly costs are reduced, there is still a lag in ABS braking response time due to the pressure load that is placed on each ballscrew piston motor by the hydraulic back pressure of the system. Thus, as with ABS braking in standard hydraulic braking systems, ABS response times would be greatly improved in dry interface braking systems if the pressure load on the motor could be controlled during ABS apply and reapply modes.
Thus, given the above-noted disadvantages of prior art automobile braking systems, it is desirable to have a hydraulic control system that is highly responsive to driver braking input and that controls pressure load on a motor during ABS braking events to improve the hydraulic system response time.
The present invention is an improved control for a hydraulic system. While the improved control disclosed herein is useful in many hydraulic system applications, it is particularly useful in automobile braking systems. The improved control of the present invention preferably includes the use of a normally-open solenoid valve that closes when a desired pressure is sensed in the associated hydraulic system. The closing of the normally-open solenoid valve holds the pressure at the wheel brake assembly at a desired level while simultaneously relieving pressure load on the motor being used to create the hydraulic pressure during a release cycle and deadheading the motor on an apply cycle, which may be the ABS motor in standard braking systems or the ballscrew piston motor in dry interface systems. The control of the pressure load allows the motor to return more quickly to a neutral position, which allows the system to reset more quickly. Accordingly, the use of the normally-open solenoid in this application increases the efficiency, accuracy and system response of the hydraulic braking operation.
Additionally, the improved control of the present invention is also useful for relieving motor load during long, constant brake pressure stops, such as those that occur at a stoplight, thereby lowering the motor current requirement. This increases the electrical efficiency of the system, which is particularly useful in electric vehicles, and reduces motor wear. Further, the improved control of the present invention is useful for holding brake pressure constant in specialty situations such as hill holder braking applications.
In one embodiment of, the present invention, a normally-open solenoid is combined with a motor-driven ballscrew and piston assembly in a dry interface braking system in order to achieve greater control of the brake apply and release cycles during ABS braking. Additionally, in a preferred embodiment, the solenoid can be used to lower wear on the motor and decrease the amount of energy required to keep the brakes in an applied position during long stops. In this embodiment, the normally-open solenoid valve is located between the hydraulic output to the brake calipers and the ballscrew piston assembly. Accordingly, the pressure of the hydraulic fluid applied to the calipers can be held constant by closing the solenoid valve when a desired pressure is detected by a pressure sensor. Thus, by coordinating the operation of the solenoid valve with the driver input and/or ABS system using an electronic controller, the desired pressure at the brake assembly can be maintained while pressure load on the ballscrew piston motor is varied.
In an alternate embodiment, a normally-open solenoid valve is used in a standard all hydraulic braking system with motor-based ABS. The solenoid is positioned between the master cylinder and the brake calipers. The solenoid is then actuated between on and off positions to assist the motor that is used to modulate apply and release cycles during ABS braking in order to improve response times in a similar manner as described above with respect to dry interface systems. Additionally, the solenoid valve may be closed in hill holder situations to hold the pressure to the brakes constant when this situation is sensed by the braking system.
It should be noted that while almost any solenoid valve, or even other types of fluid valves, may be used to regulate fluid flow to a pressure actuated mechanism in accordance with the present invention, it is preferred to use a quick acting valve which minimizes fluid flow impedance during xe2x80x9copenxe2x80x9d times. Accordingly, by using a fast-acting, low-flow impedance valve, the hydraulic system response is optimized.
An exemplary control sequence of a vehicle braking event illustrating one use of the improved control for a hydraulic system of the present invention in a dry interface braking system is as follows. First, a signal is sent to the electronic brake controller indicating whether or not the automobile is moving. If the signal indicates that the automobile is moving, the brake controller determines if ABS is active. If ABS is active, the brake controller sends a signal indicating the condition of the ABS hold. If the ABS hold is active, meaning that the brake controller is sensing a condition wherein it is desired that brake pressure be held constant, the solenoid valve is closed to maintain the current pressure at the brakes. Then a signal is sent to the ballscrew piston motor to reverse motor direction until the motor is returned to a neutral position. The routine is then exited and started again. However, if the brake controller senses that ABS hold is not active, the brake controller regulates current to the ballscrew piston motor while the solenoid remains open, thereby pressuring the brakes to a desired output. The routine is then exited and started again.
In contrast, if the initial signal received by the brake controller indicates that the automobile is not moving, the status of the driver braking command is checked. If the driver braking command is constant, the solenoid valve is closed, thus holding the brake pressure at the desired constant level. The ballscrew piston motor is then turned off and the routine is exited and started again.
Accordingly it is an object of the present invention to provide an improved control for a hydraulic system, an improved control suitable for automotive vehicle braking systems, that is highly responsive to driver braking input, that controls pressure load on a motor/actuator during ABS braking events, and that improves the hydraulic system response time.
Other objects and advantages of the present invention will be apparent from the following description, the accompanying drawings, and the appended claims.